Scientist already produced a virus, that never existed before, just because they could.
Now they're able to produce sperm cells from stem cells that could potentially come from anyone. Want to bet that someday, some woman will have sperm produced from her own stem cells, so that she can be impregnated by herself?

I don't think it's unethical. There are some serious ethical questions about using these sperm cells to impregnate a woman or create a baby, but just creating the sperm cells in the first place, to show it can be done, to learn more about how to use stem cells, I don't think anyone has a problem with that.